The field of dentistry routinely requires the manipulation of a high-viscosity, adhesive material. In one application, a dimethacrylate composite is used to fill oral cavities. Dimethacrylate composite esthetic restorative materials have been the subject of considerable research since their introduction in 1962. They represent the current state of the art in the drive to develop restorative materials that have the appearance of natural teeth and do not contain mercury. Composite restorations have been shown to have good clinical performance, particularly for application in the anterior portion of the mouth, where the mechanical stresses to which the restoration is subjected are comparatively low. Posterior, load-bearing applications are generally more problematic. It has been demonstrated, however, that when properly placed, composite dental materials can produce posterior restorations with excellent longevity. Proper placement refers to placement and packing of dental restoratives in such a way that firm adhesion to the dental substructure is achieved, with a minimum number and size of voids due to air entrapment, and minimization of overall porosity.
Due to their strong adhesive properties, composite restorative materials can stick to dental instruments and be difficult to manipulate, causing the practitioner to introduce flaws and voids into a restoration that may cause a degradation of nominal mechanical and clinical properties. During a dental restorative procedure, after a composite is initially placed inside an oral cavity, a step is routinely required in which pressure is applied to the composite to remove entrapped air and to ensure adaptation and a firm bond of the composite to the cavity walls. This step is referred to as "packing," and is fraught with difficulties caused by the extreme stickiness of the restorative materials. In general, the composite sticks to the packing tool when the tool is withdrawn from the restoration. As shown in the illustration of a restored tooth 10 in FIG. 1, the resulting strain on the composite 12 during the removal phase can cause air entrapment and disbanding of the composite 12 from the inner walls 14 of the cavity.
The sticking of the restorative material to the tool is known as adhesion, or adhesive "tack," the elimination of which has been the focus of considerable efforts over the last two decades. Presently, packing tools commonly used by dental practitioners include instruments, or "pluggers," with a variety of geometries made of materials such as plastic, Teflon.RTM., and stainless steel, as well as injection syringes.
The development of low-viscosity, flowable composites have effectively removed the packing step from the clinical procedure. However, in order to achieve low viscosity, these composites are manufactured with a lower volume percentage of filler particles in their matrix, and therefore do not have the mechanical strength of high-viscosity composites. Flowable composites are recommended for small class III or class V cavities, but are not recommended for large cavities or for any application subject to wear.